Various methods for gene transfer to cells are known. For example, a method using a virus vector, a method using a specific receptor, or a cell fusion method is known as a biological method; a microinjection method, an electroporation method, a gene gun method, or an ultrasound-mediated gene transfer method is known as a physical method; and a calcium phosphate coprecipitation method, a liposome method, a lipofection method, a DEAE dextran method, or an alkali metal method is known as a chemical method. The lipofection method is a method which involves forming a liposome-DNA complex by the electric interaction between a lipid bilayer membrane vesicle consisting of a positively charged lipid or the like (liposome) and DNA to be transferred and incorporating the complex into host cells by phagocytosis or membrane fusion. The lipofection method is distinguished from the liposome method in that DNA is not necessarily encapsulated in the liposome. Generally, when gene transfer is carried out, the lipofection method is first performed due to its simplicity. The lipofection method is relatively widely used because it has the merit of having high versatility and simplicity and being suitable for the treatment of many samples. However, the lipofection method also has the demerit of being inferior to the electroporation method or the method using a virus vector in terms of gene transfer efficiency. The method using a virus vector has the merit of having high gene transfer efficiency, but has, for example, problems of 1) it taking a time of as much as about 1 week until a target gene is placed in a virus vector, followed by transferring the virus vector to virus-producing cells, then producing viruses containing the target gene, and infecting cells with the viruses, 2) requiring facilities for handling a virus vector, 3) requiring technical personnel sufficiently skilled for handling a virus vector, and 4) sometimes inserting the virus vector into the somatic genome and leaving it there. Thus, if a method can be developed for efficiently transferring a gene without using a virus vector, it represents a useful method.
The present inventors have been hitherto studying for improving efficiency of gene transfer to yeast. For example, Patent Document 1 describes a method for transforming yeast, comprising directly mixing a yeast culture with a solution containing DNA to be transferred to the yeast cells, an alkali metal ion, and polyethylene glycol (PEG) for transformation. Patent Document 1 also describes the further use of carrier DNA or carrier RNA in the above mixed solution. However, the method of the Patent Document 1 is a method for yeast cells and based on an alkaline metal method. The method of the present invention is entirely different from the method of Patent Document 1 because it is a method for mammalian cells and based on the lipofection method. In addition, Patent Document 1 teaches nothing about tRNA. Because yeast cells have cell walls, the lipofection method is not commonly used.
Incidentally, Patent Document 2 describes a transformation method having a high gene transfer efficiency, using mammalian cells as host cells. The transformation method of Patent Document 2 is characterized by simultaneously transferring a first vector comprising a mammalian replication initiation region and a mammalian nuclear matrix association region and a second vector comprising a promoter functioning in mammalian cells and a target gene encoding a secretory protein to mammalian cells so that the mole ratio of the first vector to the second vector ranges from 0.3 to 5. Patent Documents 1 and 2 also disclose that the use of a polyethylene glycol having a particular molecular weight improves gene transfer efficiency. However, both documents teach nothing about the use of tRNA in gene transfer to mammalian cells.